Typically a coach taking a year off is a sign they want out....when have you ever seen a coach take a year off and come back to the same team...see Sean Payton.
Again this proves nothing. My guess is still that he chose to leave. The Bills wouldn't have waited so long to fire him.
He decided he wanted out, all the coaching vacancies were filled. So he took a year off let his contract expire and then started looking for new opportunities this offseason.